1. Field of the Invention
The present invention generally relates to regeneration of an optical signal in an optical signal transmission system. More particularly, the present invention relates to an improved optical signal regenerator configuration for an optical signal transmission network.
2. Background of the Related Art
Although a number of data transmission technologies are widely used throughout the world, optically based signal/data transmission systems are quickly becoming a preferred transmission method, as optical transmission systems provide performance and economic advantages not generally available from conventional transmission systems and methods. For example, optical signal transmission systems and methods generally provide a comparative signal carrying capacity that is unmatched by conventional transmission systems and methods and, therefore, optical signal transmission networks are an attractive replacement for conventional long and short haul-type transmission systems.
In long haul-type optical transmission systems, for example, a digital optical signal is generally generated at a first location and transmitted to a second location through an extended optical transmission medium, such as a fiber optic cable network, for example. However, in view of the long haul transmission distance, the optical signal may be refreshed one or more times during the transmission process through the network or medium, such as whenever a digital optical signal is generated, transmitted, switched, multiplexed, demultiplexed, or otherwise processed in a transmission medium or network. The optical signal invariably is subject to some degree of distortion. This distortion may result from noise in the system, interference from other signals in the system, the physical properties of the transmission medium, physical properties of the various inline elements of the transmission system, i.e., amplifiers, filters, etc., and other sources known to cause distortion in an optical signal transmission system. Distortion in optical transmission systems is typically cumulative, and therefore, if the original optical signal being transmitted through the system is not periodically refreshed or restored to a signal equal to or approximating the original signal, then the optical signal may become riddled with errors or become completely incomprehensible when received at the receiving end.
In order to address this issue, optical regenerators are typically utilized to provide restoration of optical signals in an optical signal transmission network during the long haul transmission process. Optical regenerators may be positioned at predetermined distances along the long haul optical signal transmission medium, and may operate to regenerate the optical signal before the signal degrades beyond recognition. Typically, optical regenerators are complicated and expensive opto-electronic devices, wherein an optical signal is converted into an electrical signal, the electrical signal is then amplified and restored/filtered to approximate the original signal, and then the electrical signal is converted back to an optical signal and is transmitted through the next segment of the haul distance to either another regenerator or a receiver, for example. However, currently, all-optical regenerators are emerging as a technology that that does not require conversion of the optical signal into an electrical signal for the signal restoration process. In all-optical regenerators the received optical signal itself is regenerated without conversion to an electrical signal. In a published letter entitled “10 Gbit/s Soliton Data Transmission Over One Million Kilometers,” M. Nakazawa, E. Yamada, H. Kubota, and K. Suzuki, Electronics Letters 27, 1270–1272 (1991) a method is suggested in which in-line amplitude modulation of a signal at the bit-rate frequency acts as a regenerator distributed over the transmission line (local clock recovery is required in conjunction with this technique). Further, a published letter entitled “All-Optical Signal Regenerator,” J. K. Lucek and K. Smith, Optics Letters 13, 1226–1228 (1993) demonstrates a method for all-optical data regeneration (also requiring local clock recovery) utilizing a nonlinear optical loop mirror as described in “Nonlinear-Optical Loop Mirror,” N. J. Doran and D. Wood, Optics Letters 13, 56–58 (1988).
Additionally, commonly assigned U.S. Pat. No. 6,141,129 to Mamyshev, for example, generally discloses an all optical signal regeneration apparatus and method that may be utilized for all-optical regeneration of return-to-zero (RZ) data streams. The apparatus and method disclosed by Mamyshev may be applicable to soliton as well as non-soliton pulses and is generally capable of operation without the use of local clock recovery, which was also required by previous optio-electronic regenerator systems. Mamyshev accomplishes these advantages through utilization of the effect of self-phase modulation (SPM) of an optical data signal in a nonlinear medium (NLM) to provide a spectral broadening of individual data pulses. The broadened pulses are subsequently bandwidth filtered, and therefore, only the pulses within a selected bandwidth range centered at a predetermined frequency pass through the filter. The predetermined frequency is generally shifted with respect to the input signal carrier frequency, and therefore, since the degree of broadening of an optical pulse passing through the NLM is generally a function of the initial intensity of the pulse, a large portion of the noise in signal zeros or null values will then possess insufficient intensity to cause the requisite amount of spectral broadening to encompass the selected filter bandwidth centered around the predetermined frequency. As a result thereof, a portion of the noise is suppressed. Conversely, noise (amplitude fluctuations) in signal ones or set values generally possess sufficient intensity to cause the requisite amount of spectral broadening to encompass the selected filter bandwidth centered around the predetermined frequency. As a result thereof, the portion of the spectrally broadened pulse contained within the bandwidth centered around the predetermined frequency is generally passed through the filter. Thus, the end result is a regenerated and filtered signal.
Although the method and device of Mamyshev generally provides an inexpensive and easily produced first generation optical regenerator, Mamyshev's optical regenerator is prone to passing noise through the filter when the noise is centered proximate a filtering frequency and has a sufficient magnitude. When this type of noise is passed through the optical regenerator, false set values are produced in the regenerated output. Therefore, there is a need for an improved optical regenerator capable of filtering noise and producing a regenerated output that accurately represents a source optical signal.